Power systems are going through a paradigm change from centralized generation to distributed generation. More and more distributed energy resources (DERs), including renewables, electric vehicles, and energy storage systems, are being integrated into the grid through power electronic converters operated as inverters. The majority of loads will also be connected to the grid through power electronic converters operated as rectifiers. As a result, the fundamental challenge behind the paradigm change of power systems is that future power systems will be power electronics-based instead of electric machines-based. This presents unprecedented challenges to the stability and reliability of power systems. It has been shown that the SYNDEM (synchronized and democratized) grid architecture is able to unify and harmonize future power systems via operating these converters as virtual synchronous machines, bringing compatibility to current power systems and enhancing stability, reliability and resiliency. This solves the challenges at the systems level and enables millions of power electronic converters to work together. However, there are still a lot of challenges at the equipment level.
The design of power electronic converters is still a challenge. It requires multidisciplinary knowledge in power semiconductor devices, topology, electrical and electronic circuits, thermal management, mechanical design, control theory etc. Among them, adopting the right topology is vital for power density, EMI, thermal management and many other issues. Providing a common reference point is very beneficial in this regard. For example, U.S. Pat. No. 6,271,633 B1 discloses a single-phase back-to-back converter with a common neutral leg to provide a common reference point. US 2006/0077701 A1 extends this topology to a three-phase back-to-back converter with a common neutral leg to provide a common reference point. However, the mid-point of the common neutral leg in both cases is directly connected to the common reference point, causing large current ripples and making it difficult if not impossible to control the common leg independently. Indeed, it can be seen from the control block diagrams disclosed in US 2006/0077701 A1 that the controller for the common leg is coupled with the controller for the other legs. Another critical aspect of a power electronic converter that affects power density and cost is the measurement of voltages and currents and the arrangement of the corresponding sensors. Not enough attention has been paid to this aspect in the prior art. Indeed, neither U.S. Pat. No. 6,271,633 B1 nor US 2006/0077701 disclosed how the current sensors and the voltage sensors were connected. Conventionally, bulky and expensive isolated voltage and current sensors are often used.
A common way for measuring currents is to place current sensors directly in series with inductors. When plural current sensors are used in a power electronic converter, the current sensors do not have a common point. For example, current sensors Ls1 and Ls2 in FIG. 2 of US Pub. No. 2002/0180379 are connected between the inductors L1 and the mid-points Vx1 and between L2 and Vx2, respectively. The two current sensors are not connected directly and do not have a common point. These current sensors present a high common-mode voltage and require isolation for electronic control circuitry, which increases circuit complexity and cost.
A common way for measuring voltages is to place voltage sensors across the voltage to be measured. When plural voltage sensors are used in a power electronic converter, the voltage sensors do not have a common point. For example, paragraph 110 of US Pub. No. 2004/0233688, recites with regarding to FIG. 4 there that “the voltage detecting means 70 is provided for detecting the voltage Vin at both ends of the AC power supply 1, a voltage VM (VMN) at the point M based on the potential at the point N, a voltage VP (VPN) at the point P based on the potential of the point N, and the voltage Vout.” The voltages Vin and Vout are measured based on the potential of the neutral point of the AC power supply 1 and the load 6 while the voltages VM (VMN) and VP (VPN) are measured based on the potential at the point N. The four voltage sensors are not connected directly to a common reference point. These voltage sensors require isolation for electronic control circuitry as well, which increases circuit complexity and cost.